Not Applicable
1. Field of the Invention
This invention relates to an ultrasound transmitting and receiving system, or transceiver, including an ultrasound transducer, an ultrasound waveguide and a jacket-type enclosure, said ultrasound waveguide being housed within the enclosure while the ultrasound transducer is positioned at one end of the ultrasound waveguide, whereby at said end of the ultrasound waveguide the ultrasound transducer can receive and transfer ultrasound waves from/to the ultrasound waveguide.
Ultrasound transceivers of this type are employed for instance in ultrasonic flowmeters and in vortex flowmeters. The ultrasound transducers typically used in these designs are piezoelectric crystals that are capable of generating as well as detecting ultrasonic waves.
In principle, it would be possible to equip an ultrasound transceiver with only one ultrasound transducer for generating as well as receiving ultrasonic waves. That, however, would require positioning the ultrasound transducer directly at the point where the ultrasonic waves are to be injected or detected. This would be difficult insofar as piezoelectric crystals, typically serving as ultrasound transducers as stated above, cannot be used above a certain temperature referred to as the Curie temperature. The reason is that above this Curie temperature, the crystal no longer possesses a ferroelectric or ferromagnetic phase, the very prerequisite for the piezoelectric properties of the crystal. Yet in cases where the moving fluid whose flow rate is to be measured with the ultrasonic flowmeter is very hot to a point where its temperature exceeds the Curie temperature of the piezoelectric crystal, any reliable operation requires a certain thermal insulation of the ultrasound transducer from the hot fluid. This is why ultrasound transceivers employ ultrasound waveguides that are designed to offer both best possible thermal insulation of the ultrasound transducer from the hot fluid and, to the extent possible, a loss-free and unimpeded transfer of the ultrasound signal. In that fashion, the ultrasound waveguide can inject ultrasonic waves generated by the ultrasound transducer into the flowing fluid and the ultrasound transducer can extract ultrasonic waves from the hot fluid while the ultrasound transducer is at a spatial distance from the hot fluid and is thermally insulated from the latter at least to a certain extent.
2. Description of the Prior Art
Conventional ultrasound transceivers employ ultrasound waveguides for instance of the type described in WO 96/41157. The ultrasound waveguide in that case is constituted of a plurality of very thin, mutually parallel rods whose individual diameter is substantially smaller than the wavelength of the ultrasound signal to be transferred. The rods are typically bundled close together and fitted into a tube that provides lateral support for the rods and thus constitutes an enclosure, or jacket, for the ultrasound waveguide, the result being a compact ultrasound waveguide. For an ultrasound waveguide, WO 96/41157 also describes a design where metal plates, bent in essentially circular fashion, are interleaved at a distance from one another. These, too, are housed in a tube that constitutes an outer jacket for the ultrasound waveguide. Finally, EP 1 098 295 discloses an ultrasound waveguide that consists of a rolled-up foil tightly fitted into a metal tube. To permit the transfer of ultrasonic waves in the frequency range from 15 kHz to 20 MHz, the thickness of each layer of the foil is less than 0.1 mm. The foil typically consists of a metallic material.
What the ultrasound transceivers incorporating ultrasound waveguides of the type described have in common is that the ultrasound transducer is positioned at one end of the ultrasound waveguide in such fashion that ultrasonic waves can be injected by the ultrasound transducer into the ultrasound waveguide and/or can be received by it from the latter. The typical approach involves the direct attachment of the ultrasound transducer to one end of the ultrasound waveguide, meaning physical contact between the two. Where the ultrasound waveguide is in the form of a rolled foil as described above, the ends of the ultrasound waveguide are usually welded up and butt-faced and the ultrasound transducer is mounted on this welded-up and level end face of the ultrasound waveguide.
However, the problem with ultrasound transmitters of the type described above is that ultrasonic waves generated by the ultrasound transducer enter not only the ultrasound waveguide but the jacket encasing it as well. A similar problem is encountered when the ultrasound transducer also serves to detect ultrasonic waves, i.e. as an ultrasound receiver, in which case ultrasonic waves travel to the ultrasound transducer not only via the ultrasound waveguide but by way of the jacket as well. As a consequence, when the system includes both an ultrasound transmitter and an ultrasound receiver, it is not only ultrasonic waves transmitted and received via the ultrasound waveguide that are measured but also ultrasonic waves that travel through the jacket of the waveguide. Now if on top of that the ultrasound transceiver system is installed with its jacket, for instance, into the wall of a pipe through which flows the fluid whose flow rate is to be measured, such measurements will include not only the ultrasonic waves that penetrate the fluid, but also those waves that have propagated through the wall of the pipe from the ultrasound transmitter to the ultrasound receiver. This phenomenon is referred to as cross coupling or crosstalk and may lead to a heterodyning or even complete suppression of the actual measuring signal of interest.
The problem associated with this manifests itself even more when one realizes that, when ultrasonic waves are switched between two mutually different media, the coefficient of transmission will be as follows, disregarding any geometric factors:
T=4(z1/z2)/(1+z1/z2)2
where z1 and z2 are the characteristic impedances of the first and, respectively, second medium between which the transition takes place. In the case of a transition from steel to air, the aforementioned coefficient of transmission T is approximately 0.004%. This means that a significant part of the acoustic energy, 99.996% to be exact, is lost. A major portion of this lost energy reappears in the undesirable cross coupling. Accordingly, this cross coupling or crosstalk severely affects the signal-to-noise ratio of a flowmeter that operates with an ultrasound transceiver.
It is therefore the objective of this invention to introduce an ultrasound transmitting and receiving system by means of which any undesirable cross coupling or crosstalk can be largely avoided.
With reference to the ultrasound transceiver design described above, the invention achieves that objective by means of an impedance step between the ultrasound transducer and the side of the jacket facing away from the ultrasound transducer.
This impedance jump as provided for by the invention thus covers an area of the jacket of the ultrasound transmitter in which the undesirable entry of ultrasonic waves from the ultrasound transducer into the jacket is significantly attenuated. Correspondingly, an ultrasound receiver according to the invention includes an area in which ultrasonic waves impinging on the jacket of the ultrasound receiver and conducted to the ultrasound transducer are significantly attenuated. This is because in both cases the ultrasonic waves travelling through the jacket must pass the impedance step, either as they come from or move toward the ultrasound transducer, with the magnitude of the impedance jump determining the degree of attenuation of the intensity of the ultrasonic waves essentially along the formula shown above for the coefficient of transmission.
An impedance step as provided for by the invention in the jacket of the ultrasound transceiver can be implemented in various ways. For example, in one preferred embodiment of the invention, the jacket is positioned at a distance from the ultrasound transducer. In this fashion, an air gap is created next to the end of the jacket facing the ultrasound transducer which, as explained above with reference to the coefficient of transmission, results in a very substantial attenuation. In this connection, in a further preferred refinement of the invention, that gap is filled with a material that is different from the material of the jacket and of the ultrasound transducer. This is particularly desirable when a geometrically uniform transition from the ultrasound transducer to the jacket of the ultrasound transceiver is required, prohibiting the use of a recess.
As an alternative approach, another preferred embodiment of the invention provides for an impedance step within the jacket itself. This impedance step is attainable, for instance, by means of a recess in the jacket. As an example, the recess may be in the form of one or several drill holes. These may be blind holes or even through-holes extending through the entire thickness of the jacket. The recess may also be in the form of a preferably circumferential groove. The groove as well may extend through the entire thickness of the jacket, or only through part of the thickness of the jacket. Significantly, it is possible in this context to provide a groove whose depth varies along its circumferential extent.
If the impedance step is created in the jacket itself by means of a recess within the jacket, that recess can again be filled at least in part with a material that is different from that of the jacket and that of the ultrasound transducer.
In another preferred implementation of the invention, the ultrasound transceiver is employed in a measuring device with a vessel containing or conducting a fluid such as a gas or a liquid. A vessel containing or conducting a fluid would primarily be a tank or a pipe. An example of a vessel conducting a fluid would be a pipe equipped with an ultrasound flowmeter. In a flowmeter of that type, a preferred embodiment of the invention provides for an ultrasound transmitter and/or an ultrasound receiver, both as described above, to be mounted in the wall of the vessel above the area of the jacket that faces away from the ultrasound transducer. Thus, in the case of an ultrasound transmitter, only that part of the jacket is in contact with the wall in which the ultrasonic waves have already been significantly attenuated. The undesirable crosstalk via the wall of the vessel is substantially reduced. Conversely, in the case of an ultrasound receiver ultrasonic waves may enter by cross coupling via the wall of the vessel into that area of the jacket above which the ultrasound receiver is mounted in the wall of the vessel. But because of the impedance jump that separates this area from the region leading to the ultrasound transducer, the latter will pick up only a substantially reduced crosstalk component.
It would be entirely possible to mount the ultrasound transmitter and/or the ultrasound receiver directly in the wall of the vessel. In a preferred embodiment, however, a flange is provided for attaching the ultrasound transmitter and/or the ultrasound receiver, with additional crosstalk attenuation attainable by means of an attenuator ring installed between the flange and the jacket of the ultrasound transmitter and/or ultrasound receiver. In selecting the material for the attenuator ring an effort should again be made to obtain the largest possible impedance step. Since the jacket of the ultrasound transceiver typically consists of a metal and the wall of the vessel on its part is usually made of a metal, the material chosen for the attenuator ring would typically be a type of plastic or rubber.